This invention relates to devices for providing information, in the form of electric signals, about the position and pressure of a selected touch point on a touch sensor that employs a resistive membrane. In particular, these devices are used as man-machine interfaces.
In general computing applications, this invention can be used as a two-axis pressure-sensitive touch controller. As such, it can perform the cursor control functions of a mouse, joystick, or trackball, with the enhanced feature that the pressure signal provides an additional independent control dimension. In the paper "Issues and Techniques in Touch-Sensitive Tablet Input", 1985 ACM 0-89791-166-0/85/007/0215, which is incorporated by reference herein, William Buxton et al. discuss human-factors issues in using touch tablets for human-computer interaction, and in particular cite the need for a pressure-sensitive touch tablet.
In dedicated control applications, a single-axis pressure-sensitive touch controller can perform the functions of a traditional potentiometer of fader. When combined with an LED or LCD display, a solid-state fader can be constructed that may be as easily controlled by a computer as by a human.
A two-axis pressure-sensitive touch controller is also a versatile and sensitive controller for specific computer applications. These include musical performance controllers and video game controllers.
U.S. Pat. Nos. 4,071,691 and 4,129,747 describe a two-axis pressure-sensitive touch controller that requires the user to make physical and electrical contact between two surfaces. One surface is a resistive layer deposited on an insulating substrate, with a periodic signal applied across it. Adjacent to the resistive surface is a second surface which is a conducting plate that detects the signal from the first surface as it passes through the hand of the user. This device restricts the movement of the user because it requires the user to touch two surfaces simultaneously.
The deficiencies of the two-surface approach were later solved with U.S. Pat. No. 4,293,734, which is especially useful for touch screens that overlay computer displays. Current-to-voltage converters are connected to the corners of a resistive layer in order to provide periodic voltage sources that also measure the currents that the voltage sources provide. These currents will change as a function of the impedance presented by the user's body at the touch point.
This method eliminates the need for the second pickup plate, however it requires the position voltages to be divided by the magnitude of the total current drawn by the user's finger, which is equivalent to the sum of all the position voltages. This current is dependent upon several factors that are not easily controllable, such as the area of the electrical contact beneath the fingertip, the impedance of the electrical connection at the touch point, and the extent to which the user is grounded.
Further, a division operation in any form (i.e. digital or analog) tends to reduce the accuracy of the data, but it is especially difficult to perform true linear division using simple analog circuitry. Nonlinearity in the division operation would have the effect of coupling the pressure and position measurements, so that changes in pressure would perturb the position measurement.
An alternative approach is disclosed in U.S. Pat. No. 4,570,149, which does not require the user to make direct electrical contact with the resistive layer, and further provides position measurements that do not require division operations. A voltage supply layer is constructed by depositing closely spaced parallel conducting traces on an insulating substrate, and connecting one end of each conducting trace to a common resistive trace.
A pickup layer is constructed by depositing a conducting material on a flexible insulating substrate. The pickup layer is mounted over the supply layer, so that the two layers make electrical contact when a user presses on them. In order to form a two-axis touch pad, two assemblies each containing a supply layer and a pickup layer are superimposed such that the conductive traces in each assembly are orthogonal to each other.
One end of each resistive trace is connected to a common voltage source, and the other end is left open. Each pickup layer conducts the current from the voltage source through some portion of the resistive trace that represents the position of the touch point. This current is used in a timing circuit to charge a capacitor, resulting in a time constant that is proportional to the position of the touch point.
Another possible method for determining the touch point using this touch sensor, as described in U.S. Pat. No. 4,587,378, is to connect two different voltage sources to either end of the resistive trace, establishing a potential gradient that causes successive conductive traces to be maintained at linearly increasing voltage. A potentiometer is thus formed where the pickup layer behaves electrically as does the wiper of a potentiometer, transmitting the voltage at a particular conductive trace, which in turn is a linear function of position.
Another touch sensor is described in U.S. Pat. No. 4,897,511, which employs two resistive sheets that are brought into contact at the touch point. A constant current source is applied to one edge of the upper sheet, with one edge of the lower sheet connected to circuit ground through a resistor. The resulting voltage drops from the edges of each sheet to the touch point are detected with differential amplifiers.
Although these devices may function well, they are not pressure-sensitive, but may serve as the basis for a pressure-sensitive touch controller.
The conducting supply traces carrying the position voltages are interdigitated with conducting pickup traces that are all electrically connected, so that the supply traces and the pickup traces are deposited on the same substrate but are electrically isolated from each other. A layer of force-sensing resistor (FSR) material is deposited on a flexible insulating substrate which is mounted over the voltage trace layer so that when a user presses on the assembly, the FSR will shunt a supply trace with an adjacent pickup trace.
An FSR is a passive material that has the property that its electrical resistance changes in response to an applied force. Greater applied forces result in smaller resistances, and the removal of force appears as an open circuit (infinite resistance). FSR materials are described in U.S. Pat. Nos. 4,314,227 and 4,315,238. U.S. Pat. No. 4,252,391 discloses a different material that also changes its regional resistance as a function of force, and can be formed into thin sheets.
The resulting touch-sensitive sensor, called a force-and-position sensing resistor (FPSR), is manufactured by Interlink Electronics, Inc. of Santa Barbara, California, and is described in their Applications Notes IL-03, which is incorporated by reference herein, as well as in U.S. Pat. No. 4,810,992. The method suggested in this literature for measuring the touch position is based on the potentiometer method as described above.
Schematically, the FSR is electrically connected in series between the wiper of the potentiometer (the pickup traces) and the supply traces. In order to measure the resistance of the FSR so that the touch force may be determined, the terminals at the voltage supply traces are reconfigured so a constant potential is established on all of the supply traces, and a fixed pulldown resistor is connected from the pickup traces to ground. The voltage across the pulldown resistor is measured, which is a function of pressure.
Although the FSR changes its resistance as a function of the force applied at the exact touch spot, the FPSR actually integrates the currents supplied under the entire area of the touch, so that the FPSR in effect measures pressure, not the applied force.
This processing method is problematic because the FSR can have a resistance of several megohm at light touch forces, causing an extremely high output impedance when attempting to measure the position voltage. The result is that changes in the applied force can affect the measured position, even though no change in the physical touch position has occurred.
Likewise, the force measurement can be corrupted by changes in position because the supply traces will exhibit different voltage drops, depending on the touch position. Furthermore, the process of time-multiplexing between a position measurement mode and a pressure measurement mode complicates the circuit, introduces switching transients, and slows the response time.
Another difficulty arises from the construction method of two independent assemblies superimposed to measure position along orthogonal axes. When a light force is applied, it often happens that the upper assembly makes contact before the lower assembly, with the result that only the upper position measurement is valid.
Also, FSRs have a logarithmic resistance response to applied force that is only approximately linear when a small range of force is used. When the range is extended to light forces, the response has a noticeable logarithmic feel, rather than an expected linear response.
The common workaround for these deficiencies has been to set high pressure thresholds in software post-processing, in order to ensure that only the smaller FSR resistance values are used to determine whether a touch has occurred. The result is that only a restricted range of forces is allowed, reducing the useful range of touch sensitivity of the sensor.